1. Field of the Invention
The present invention relates to an image-taking lens unit. More particularly, the present invention relates to an image-taking apparatus, such as a digital camera or a digital appliance equipped with an image capturing capability, that captures an image of a subject with an image sensor, and to a slim, high-zoom-ratio image-taking lens unit suitable for use in such an image-taking apparatus.
2. Description of Related Art
With the recent spread of personal computers, there have been increasing demands for digital cameras, which permit easy capturing of images. With this trend, increasingly slim digital cameras have been sought after, and accordingly increasingly slim image-taking lens systems have been craved for. On the other hand, some digital cameras now available on the market have achieved a dramatic slimming-down by the use of a prism disposed within its image-taking lens system. This prism bends the optical path, and thereby helps increase the flexibility with which an image sensor can be disposed inside the camera. FIG. 9 schematically shows the sectional structure of an image-taking lens unit 10 typically adopted in such a digital camera.
As will be understood from FIG. 9, the thickness Δ of the image-taking lens unit 10 depends on the outer diameter of the lens elements that are disposed downstream of where the optical path is bent. Accordingly, reducing the effective diameter of the lens elements disposed downstream of where the optical path is bent helps make the image-taking lens unit 10 slim. Since the effective diameter depends considerably on focal length, f-number, and other specifications, however, there is usually left little margin for the reduction of the effective diameter through efforts in designing. Even then, cutting off the region where the light relevant to image taking does not pass makes it possible to reduce the lens outer diameter in particular directions. This is because common image sensors have a rectangular image-sensing region, and thus, in a lens element disposed away from an aperture stop, the region where the light relevant to image taking passes is not circular. From this viewpoint, in the image-taking lens unit proposed in Patent Publication 1 listed below, a first and a second lens element disposed downstream and upstream of the bending of the optical path are given non-circular outer shapes. This minimizes the distance between those lens elements, and thus helps make the image-taking lens system slim.
Patent Publication 1: Japanese Patent Application Laid-open No. 2003-161878
Generally, molding a lens element in a non-circular shape with resin tends to result in larger astigmatism (that is, differences in curvature among different directions across the lens surface) than molding one in a circular shape. In the case of the first and second lens elements disclosed in Patent Publication 1, astigmatism tends to appear in the directions corresponding to the longer and shorter sides of the image-sensing region. Molding a lens element in a non-circular shape with optical glass is difficult, and, when one is molded of glass, it needs to be given a non-circular shape through after-processing. This leads to higher cost. Moreover, in the image-taking lens unit disclosed in Patent Publication 1, the axial ray of the light that passes through the first and second lens elements has a comparatively large height. Thus, giving the first and second lens elements non-circular outer shapes result in increased astigmatism in the directions corresponding to the longer and shorter sides of the image-sensing region.